Does he have diabetes? I believe that 40+yr old men who are diabetic are automatically offered viagra on prescription for free. Maybe he just hasn't mentioned it out of embarrassment - although that wouldn't account for all 4 tabs being used (when you've only had sex once).
Doldare- I know what you are saying but I can't find evidence of an affair. He usually pays for stuff by card and there are no questionable items on online banking. I can check his work and personal email via his phone which I know the password to. He's not possessive or secretive over his phone either. Internet history is also normal-no porn, chat sites etc.
Hmm.. can't really put it down to exercising lots as that tends to increase libido. Some red flags, but you know him and seem sure about him. Wardrobe changed a bit too? More care in appearance? How convinced are you? Maybe a bit more thought and critical observation required, or a mid-life crisis?
Viagra's not available on the NHS, so he'll have had to pay for a private prescription. Does that appear on his credit card statements? (In a chemist's shop, or Boots, I guess) If not, he's hiding the purchase from you. I can't see why a man would take Viagra for masturbation at all. You're either in the mood or not, and planning a wank for an hour's time seems improbable.
Does he have erectile issues? I can't think of a single legitimate reason why a DH would take 4 viagras when not with his Dw. I'd say nothing. Or alternatively have an insipid conversation about 'a friend' whose DH used Viagra and see if he mentions it. Spidey senses going awry I'm afraid
I'd ask in a lighthearted way. "You'll never guess what I found in the car...." and take it from there. You've been married for 10 years and have 3 kids. I'm sure you can both cope with a bit of embarrassment. It's certainly better than rifling through his e-mails etc.